Electromechanical brakes have been known for some time. U.S. Pat. No. 5,788,023 discloses a disc brake for a vehicle which can be actuated electrically and whose brake linings can be pressed against the brake disc with the aid of an electric motor. The electric motor transmits its actuation force, via a so-called planetary rolling-contact threaded spindle, onto an axially displaceably mounted piston which interacts with the brake lining.
U.S. Pat. No. 5,829,557 discloses another vehicle disc brake which can be actuated electrically and whose brake linings can, in turn, be pressed against the brake disc by means of an electric motor serving as an actuator. The electric motor comprises a spindle gear mechanism and, by means of a spindle element which can be of different designs, is connected, in the direction of displacement of the brake linings, to an axially displaceable piston which acts on a brake lining. In this patent, there is optional provision for the use of an additional gear mechanism for converting the torque and rotational speed.
A recognized disadvantage associated with conventional brakes with an electric actuator is the high actuator force which has to be applied in order to achieve a sufficient braking effect. The necessary high actuator force and the resulting large power demand of the actuator make it necessary to employ very large drive units, usually electric motors, which have large torques, and are also heavy and expensive. The result of this is that electromechanical brakes have, to date, not become widespread as vehicle brakes, for example.
In order to decrease the energy consumption of the brake actuators, so-called self-enforcing actuators have been proposed. Examples of such self-enforcing brakes can be found in U.S. Pat. Nos. 4,653,614, 4,852,699, 4,946,007, 4,974,704, 5,012,901, and 6,318,513 and in U.S. Patent Application Publication No. US 2005/0247527 (which is hereby incorporated by reference herein in its entirety). A self-enforcing brake works according to the principle that the actuation force amplifies itself. The friction force between the brake linings and the brake disc give rise, with help of a self-enforcing mechanism, to increased force against the brake linings and brake disc. This increased force gives, in turn, rise to increased friction force. Hence, it is possible to produce and control large braking forces by applying relatively moderate actuator forces.
More specifically, and referring specifically to the prior art self-enforcing brake actuator shown in FIG. 1, for brake application, an actuator force which is substantially transverse to the brake disc 1 (or in other words substantially axial) is applied on the ramp plate 2 in a way to be described, until contact between the brake pad 3 and the disc 1 is established. By means of the friction force, the ramp plate 2 is transferred in the rotation direction of the disc 1, so that the rollers 5 roll up the relevant ramps 2′ and 4′ and an application force is accomplished without applying any external brake force besides the actuator force. In other words, the brake has a self-servo effect or is self-enforcing. The application force may be controlled by the actuator force, which may be positive or negative, i.e., acting in a brake applying or brake releasing direction.
The disc brake shown in FIG. 1 is arranged in a disc brake caliper 6 in a way well known in the art. The caliper 6, which is placed astraddle of the brake disc 1, is only very schematically illustrated by shaded areas indicating attachment or support portions. The ramp bridge 4 is connected to the caliper 6 by means of two adjustment screws 7 in two threaded bores in the ramp bridge 4.
The mechanism for creating the actuator force for service braking is now to be described. An electric motor 8 can rotate a drive shaft 9 in either direction over a transmission unit 10. A bevel gear 11 supported by an arm 12 from the ramp bridge 4 can be rotated by the shaft 9 but is axially movable thereon by a splined engagement. The bevel gear 11 is in driving engagement with a bevel gear disc 13 rotationally supported by the ramp bridge 4. Eccentrically connected to the bevel gear disc 13 is a crank rod 14, which at its other end is rotationally connected to the ramp plate 2.
By turning the bevel gear disc 13 in either direction by means of the bevel gear 11 from the motor 8, the position of the ramp plate 2 in relation to the ramp bridge 4 can be set. The actuator force is transmitted by the crank rod 14. When a frictional engagement between the brake pad 3 and the brake disc 1 has been established, an application force amplification will be accomplished by the rollers 5 climbing its ramps 2′ and 4′ in response to the tangential movement of the ramp plate 2 caused by the frictional engagement with the brake disc 1. The application force may be accurately controlled by rotating the motor 8 in either direction. The adjustment screws 7 have the purpose of adjusting the position of the ramp bridge 4 in relation to the wear of the brake pad 3 (and the corresponding brake pad on the opposite side of the brake disc 1). The synchronous rotation of the adjustment screws 7 is performed by suitable transmission means, such as a chain 15, driven from the transmission 10 unit in a way not further described.
While the self-enforcing brake shown in FIG. 1 and described above provides clear advantageous over prior designs, it will be recognized by one skilled in the art that it includes no park lock (i.e., parking brake) functionality. Thus, such brakes typically require a separate parking brake, which obviously is disadvantageous from standpoints of added cost, complexity and weight. Attempts have been made to obviate these disadvantages by designing electromechanical brake actuators with integrated park lock functionality, such as by providing spring bias means for applying the brake when parked, and/or by incorporating a second electrical motor or a separate hydraulic or pneumatic system for applying the parking brake when desired.
However, while some of these more recent designs do have advantages over designs incorporating a separate parking brake, they still suffer from many of the same disadvantages, as well as others. The applicant of the present invention has recognized that it would be far more desirable for a brake actuator to employ a single electrical motor which is used to actuate the service brakes, to compensate for pad wear and to apply the park lock functionality.
What is desired, therefore, is an electrically actuated brake assembly which is lower in cost, weight and complexity as compared to known assemblies, which includes integrated pad wear compensation functionality, which includes integrated park lock functionality, which can be used with both self-enforcing and non self-enforcing brakes, and which employs a single electrical motor to actuate the service brakes, to compensate for pad wear and to apply the park lock functionality.